A7.2 - Contactless Measurement of Metallic Targets by a Specially Designed Position Sensor

SENSOR+TEST Conferences 2009
2009-05-26 - 2009-05-28
Congress Center Nürnberg
Proceedings SENSOR 2009, Volume II
A7 - Magnetic Sensors II
U. Kindler - Fachhochschule Luebeck, Luebeck, Germany
47 - 52


It is a well known technical problem to fix a separate magnet as target for measurement. The innovation of the position sensor presented here is a special geometric form of flat coils which are arranged as a couple. These coils are electrically interconnected to an alternating current half bridge. The metallic target moves over the coils in a non contacting manner and in a defined working distance. The only condition for the target is to be metallic. Of course measuring magnetic targets induce bigger measuring signals. But it is not necessary for the target to be magnetic.
The flat coils produce eddy currents in the metallic target. These eddy currents damp the flat coils. Caused by the geometric form of the flat coils, the flat coils are damped in the adversary direction. The ac half bridge converts the difference caused by the damping into a voltage signal. The voltage signal is amplified. As a result (after this) the phase is correctly rectified. We get a linear relation between the movement of the target and the output signal. Angle movements can be measured by flat coils which are arranged in a circle. A ferrite plate is fixed on the back of the flat coils. The ferrite plate amplifies the measuring effect and furthermore protects the flat coils against disturbing objects on their backside. With this sensor, moving mechanical components can be measured in an easy way. In addition to this, position sensors can be applied for the identification of coins. The coins have to be moved across the sensor. As a result different coins induce specific characteristic curves. The curve characteristic depends on size and material of the coin. All these results indicate an universal approach to a whole range of applications.